Water absorbent resin (SAP/Super Absorbent Polymer) is a polymer gelling agent which is swellable with water but insoluble with water. The water absorbent resin is widely used, mainly disposably, for absorbing products such as disposable diapers, sanitary napkins, etc., and further for an agriculture/horticulture water retaining agent, an industrial waterproofing agent, and the like. For such water absorbent resin, many monomers and hydrophilic polymers have been proposed as raw materials. Especially, water absorbent polyacrylic acid (salt) resin in which acrylic acid and/or its salt is used as its monomer is most popular in industries because of its high water absorbing ability (Non-Patent Literature 1).
The water absorbent resin is produced via a polymerizing step, a drying step, and if necessary, a non-dried matter removing step, a grinding step, a classifying step, a surface cross-linking step, and/or the like (Patent Literatures 1 to 5, and 50). Meanwhile, the water absorbent resin is required to have many functions (properties) in order to cope with functional sophistication of disposable diapers which are one major application of the water absorbent resin. More specifically, the water absorbent resin is required to satisfy many properties such as, not only a high water absorbing coefficient, but also gel strength, water soluble content, a water absorbing rate, an absorbency against pressure, permeability potential, particle size distribution, an anti-urine property, an anti microbial property, impact resistance (an anti-damage property), fluidity, an deodorant property, anti-coloring (degree of whiteness), low dustiness, etc. Therefore, many cross-linking techniques, additives, modifications in steps in the production, etc. have been proposed.
Among these properties, the permeability potential is considered as a more important factor in association with a recent increase (for example, 50 wt % or more) in an amount of the water absorbent resin in disposable diapers. Furthermore, methods and techniques for improving permeability potential against pressure and permeability potential without pressure, such as SFC (Saline Flow Conductivity, see Patent Literature 6) or GBP (Gel Bed Permeability, see Patent Literatures 7 to 9), have been proposed.
Various combinations of a plurality of parameters (including the permeability potential) of the properties have been also proposed. There have been known a technique for defining impact resistance (FI) (Patent Literature 10), a technique for defining, for example, a water absorbing rate (FSR/Vortex) (Patent Literature 11), and a technique for defining the product of liquid diffusivity (SFC) and core absorption quantity after 60 minutes (DA60) (Patent Literature 12).
As the method for improving the permeability potential such as SFC and GBP, there have been known a technique for adding plaster before or during polymerization (Patent Literature 13), a technique for adding spacers (Patent Literature 14), a technique for using a nitrogen-containing polymer having 5 through 17 [mol/kg] of nitrogen atoms which can be protonated (Patent Literature 15), a technique for using polyamine, and polyvalent metal ions or polyvalent anions (Patent Literature 16), a technique for covering, with polyamine, water absorbent resin having a pH of less than 6 (Patent Literature 17), and a technique for using polyammonium carbonate (Patent Literature 18). In addition, there have been known a technique for using polyamine having water soluble content of not less than 3%, and a technique for defining a suction index (WI) and gel strength (Patent Literatures 19 through 21). There have been also known techniques for using polyvalent metal salt while controlling, during polymerization, methoxyphenol that is a polymerization inhibitor, in order to improve coloring and the permeability potential (Patent Literatures 22 and 23). Moreover, there has been known a technique for polishing particles so as to attain a high bulk specific gravity (Patent Literature 24).
Moreover, in addition to the permeability potential, the water absorbing rate is also a basic property for the water absorbent resin. As one method for improving the water absorbing rate, a technique to increase a specific surface area in order to attain a greater water absorbing rate is known. More specifically, a technique for controlling to attain fine particle diameters (Patent Literature 25), techniques for granulating fine particles with a large surface area (Patent Literatures 26 to 28), a technique for freeze-drying a hydrogel to cause the hydrogel to be porous (Patent Literature 29), techniques for performing granulation and surface cross-linking of particles simultaneously (Patent Literatures 30 to 32), techniques for foaming polymerization (Patent Literatures 33 to 48), a technique for post-polymerization foaming and cross-linking (Patent Literature 49), etc. have been proposed.
More specifically, as to the foaming polymerization, the following techniques have been known regarding a foaming agent for treating a monomer(s): techniques for using a carbonate (Patent Literatures 33 to 40), techniques for using an organic solvent (Patent Literatures 41 and 42), techniques for using an inert gas (Patent Literatures 43 to 45), techniques for using an azo compound (Patent Literatures 46 and 47), a technique for using insoluble inorganic powder (Patent Literature 48), etc.